(1) Field of the Invention
The present invention relates to an abrasive composition for polishing a plastic article to very rapidly impart a high precision mirror surface and to a process for polishing a plastic article by using this abrasive composition. More particularly, the present invention relates to an abrasive composition for polishing a plastic article such as a spectacle plastic lens, a camera plastic lens or an optical plastic lens, which composition comprises water, an aluminous abrasive powder, a polishing accelerator such as an aluminum oxalate or aluminum lactate, and if necessary, a sedimentation preventing agent such as crystalline cellulose or colloidal alumina.
(2) Description of the Related Art
The abrasive composition customarily used for plastic articles is a slurry formed by mixing water, an aluminous abrasive and a polishing accelerator such as aluminum nitrate, as disclosed in Japanese Examined Patent Publication No. 53-3518, published in 1978.
Japanese Unexamined Patent Publication No. 62-15828 published on Jan. 23, 1987 discloses a weakly acidic or weakly alkaline abrasive composition comprising water, an aluminous abrasive and nickel acetate, a lithium compound such as lithium acetate or lithium sulfate, a sodium compound such as sodium sulfate or sodium acetate, or a magnesium compound such as magnesium sulfate or magnesium acetate.
The conventional abrasive composition comprising aluminum nitrate as the polishing accelerator is disadvantageous from various aspects. First, since the aluminous abrasive is likely to settle as a solid in the bottom of a vessel storing the abrasive composition or an abrasive slurry tank used at the polishing step, a long time is required to homogenize the slurry by stirring before the abrasive composition is used. Furthermore, for the same reason, agglomerates likely to causing scratching are easily formed. If it is intended to recycle the abrasive composition and use it repeatedly, to reduce running cost, the aluminous abrasive is apt to separate and settle in a polishing machine or a slurry conduit system and the concentration of the aluminous abrasive is thereby changed, with the result that a stable polishing performance can not be obtained. Second, the abrasive composition corrodes the polishing machine and surrounding equipment, and wear and tear of the machine and equipment and inclusion of rust in the abrasive composition often occur during the polishing operation. Third, the abrasive composition adheres to the skin or cloth of an operator, and is likely to cause problems such as hand skin irritation.
To reduce the proportion of the processing cost in the total running cost, the processing efficiency of the abrasive composition must be increased.
SUMMARY OF THE INVENTION
A primary object of the present invention is to solve the foregoing problems and provide an abrasive composition for polishing a plastic article, which composition has a high polishing performance and provide a good polished surface, maintains a good polishing property even if the composition is recycled and used repeatedly, and has a reduced corrosive action on a machine and a reduced toxicity to a human body.
The present inventors polished plastic lenses by using various organic acids and various aluminum compounds of organic acids together with water and an aluminous abrasive, and found that aluminum oxalate and aluminum lactate show an extremely high polishing activity.
In this abrasive composition, it was found that the aluminous abrasive is often sedimented, and preferably this sedimentation is prevented. The inventors considered that, for a sedimentation preventing agent to increase the viscosity of the composition but have no adverse influence on the polished surface, the sedimentation-preventing agent must be finer than the aluminous abrasive and be chemically inactive, and various additives were examined based on this consideration. As the result, it was found that crystalline cellulose and colloidal alumina are very effective as the sedimentation preventing agent, and it was also found that these two additives contribute to an increase of the polishing speed due to the physical action described below.
Based on these findings, the present invention was completed in which a plastic lens or the like is effectively polished by an aluminous abrasive powder and a polishing accelerator, preferably in combination with a sedimentation preventing agent.
More specifically, in accordance with the present invention, there is provided an abrasive composition comprising water, an aluminous abrasive, and at least one polishing accelerator selected from the group consisting of aluminum oxalate and aluminum lactate, optionally with at least one anti-settling agent selected from the group consisting of crystalline cellulose and colloidal alumina.
Furthermore, in accordance with the present invention, there is provided a process for polishing a plastic article, especially a plastic lens, in which the above-mentioned abrasive composition is used.
According to the present invention, the following effects can be attained.
(1) Aluminum oxalate and aluminum lactate have a chemical action on a plastic article and the chemical polishing effect is promoted.
(2) Crystalline cellulose and colloidal alumina increase the viscosity of the composition and are dispersed in the colloidal form in the composition to retain the aluminous abrasive, whereby sedimentation of the aluminous abrasive is effectively prevented. Furthermore, crystalline cellulose and colloidal alumina retain the aluminous abrasive in a good condition on a pad of the aluminous abrasive at the polishing step, and thus the mechanical polishing effect is enhanced.